This invention relates to an optical arrangement of autofocus elements for use with immersion lithography.
In semiconductor lithography systems in use today, automatic focusing and leveling (AF/AL) is typically accomplished by passing a low angle of incidence optical beam onto the surface of a silicon wafer and detecting its properties after subsequent reflection from the wafer surface. The wafer height is determined by optical and electrical processing of the reflected light beam. This beam passes under the last element of the projection lens. The source and receiver optics are typically mounted to a stable part of the system, close to the projection optics mounting position.
In immersion lithography, a liquid such as water fills the space between the last surface of the projection lens and the wafer. At the edge of the water, typically at the edge of the lens or supported structure near the edge of the lens, the liquid-air boundary is not well defined and is changing rapidly. It is not possible to transmit an AF/AL beam through this interface without substantial disruption and subsequent loss of signal, and hence performance.
It is therefore a general object of this invention to provide a way to introduce AF/AL beams into the liquid layer without such disruption so as to preserve the optical accuracy and stability required.
More specifically, it is an object of this invention to provide an apparatus and a method for allowing AF/AL light beams to be used as in conventional lithography without the disrupting influence of the liquid immersion boundary at the edge of the lens.